Multifunctionality in Lead-Free Rb2XTaBr6 (X = Na, Li) Double Perovskites: A First-Principles Study of Structural, Elastic, Optoelectronic, and Thermoelectric Properties

First-principles FP-LAPW calculations were used to investigate the structural, mechanical, electronic, optical, and thermoelectric properties of the double halide perovskites Rb2NaTaBr6 and Rb2LiTaBr6. Structural optimization confirmed their stable cubic phase. Elastic analysis shows they are mechanically stable, ductile, and anisotropic. Spin-polarized calculations reveal that both compounds are half-metallic ferromagnets with an integer total magnetic moment of 2 μB per formula unit, dominated by the Ta-d states. They exhibit strong dielectric response and strong UV light absorption originating from interband electronic transitions, despite their intrinsic metallic ground state. Boltzmann transport theory revealed a temperature-dependent enhancement of electrical conductivity and power factor. Both materials, especially Rb2LiTaBr6, show promise for high-temperature thermoelectric and UV optoelectronic applications.